GUEST COLUMN Moving Toward Net Zero in Pre-engineered Buildings
As an industry, pre-engineered metal build- ings are facing some stiff challenges over the next several years. I know we have all been hearing of “net-zero” and wondering how that’s possible under our current designs and practices. An executive order signed in 2009 states that by 2030 all federal buildings must be net zero. In other words, they must produce as much power as they consume. That seems like a long way off and it will only
apply to federal buildings. However, the truth is we need all that time to develop new power genera- tion sources and improved building techniques, as I’m sure the intent of the law is that private construction will also pursue this lofty, and worthy goal. This brief paper will look at the law, the way power is consumed by buildings, and some thoughts on how to reach net zero.
Executive Order 13514 On Oct. 5, 2009, President Obama signed Executive Order 13514, “ensuring all new federal buildings, en- tering the design phase in 2020 or later, are designed to achieve zero net energy by 2030.” This sounds like a daunting task, and you may ask why? But if you think about it, even if we can only achieve a 50 percent reduction, wouldn’t that be worth it? So, how much power do buildings (not houses)
in the U.S. consume? How do buildings consume that power? Where does the power come from? And, of course, what can we do to move in that direction?
How much power do commercial buildings in the U.S. consume? According to the U.S. Department of Energy, there are approximately 4.9 million buildings in the U.S., covering approximately 81.1 billion square feet. The power consumed by them is equal to that of the third biggest country in the world, consuming over 6500 trillion BTUs. This power was generated by electric- ity (77 percent), natural gas (18 percent) and other sources including coal and renewables. (This is coal used for heating purposes not electricity.) Com- mercial buildings consumed 18.9 percent of the total energy used in the U.S. federal buildings consumed another 2.2 percent of the power generated. When confronted with the size of these num-
bers, one can’t help but imagine the potential for sav- ings. Now, the Executive Order only applies to federal buildings, but what if all design and construction could attain a signifi cant reduction, of say 15 percent? This kind of change would result in many positive impacts.
8 METAL CONSTRUCTION NEWS December 2013
Table 1. Areas of Power Consumption Area of Power
Consumption Lighting Heating Cooling
Ventilation
Refrigeration Water heating
Please keep in mind that this is not a sci-
Power Consumption 20.2 16
14.5 9.1 6.6 4.3
Electronics 4.4 Computers Cooking Other*
3.6 1.4
14.5
*Service station equipment, ATMs, medical equipment, etc. Heating and cooling account for more than
30 percent of the power consumed by buildings, according to data from the U.S. Energy Information Administration. What can be done to improve this? What causes the temperature loss or gain through the building? This paper is only addressing pre-engi- neered metal buildings, so keep that in mind as you read my comments.
Heat transfer in metal buildings Pre-engineered metal buildings (PEMB) are a very cost-effective building method. These buildings achieve wide spans and rather rapid construction. However, they are not the best when it comes to heating and cooling. You know they are quite hot dur- ing the summer and cold during the winter. For years we would use 4-inch insulation with a vinyl backing to achieve an R-13 rating in the ceiling and walls. The backing looks good and provides some brightness to refl ect the light, but really does little to insulate the building. It basically works as a condensation blanket that stops the panels from trapping water between the purlin and the panel, which leads to rust. We did some simple studies in our company
building here in Denver, N.C., using a surface ther- mometer. In the summer, a college intern working for us kept an hourly log for one week. We looked at the relationship of the outside temperature to the component temperature inside the building. Average temperatures for a one-week period are given below in degrees Fahrenheit.
Table 2. One-week average temperatures of structural members of an unconditioned Galvalume R-13 roof. 7 AM 8 AM 9 AM 10 AM 11 AM 12 PM 1 PM 2 PM 3 PM 4 PM 5 PM
Exterior Rafter
Eave Strut Purlin
Roof Panel
70 72 81 82 78 78
74 77 81 81 87 84
94 88 92
83 96 96
85 87 97
90 104 98 101
106 107
91
108 108
117 113 120 120 122 90
101 102
80 81 87 91 99 104 105 108 110 112 104 70 74 85 95 100
123 Percent of Total
entifi c test. We were merely looking for ideas and ways to become better sales people for our product. There is a lot of information available by people who have the knowledge and resources to do this type of testing. But if you look at the numbers, you can draw
some interesting conclusions. Obviously, the Galvalume screw down roof panel gets very hot. But even though a 4-inch layer of insulation is between the panel and the purlin, almost all that heat has transferred into the building. Not only that, the heat has transferred to the rafter as well. These materials, inside the building, are much hotter than the conditions outside. Also, notice the components are warmer than the exterior temperature in the morning, meaning the heat is stored, or at least unable to fully dissipate before the next day. This also tells you that the screws and bolts are conduits for the heat. Granted, a screw down roof with 4-inch insula-
tion is the worst-case scenario. And of course, none of these fi ndings are new or surprising. But what difference do innovations such as a standing seam roof and R-30+ insulation make? We recently constructed a school which was
a pre-engineered building, approximately 20,000 square feet with a 3-in-12 pitch roof, trapezoidal standing seam in dark blue, and we used the Simple Saver System by Thermal Design, Madison, Neb., with a layer of R-19 and R-13. We conducted the same test but did not take as many readings. The results are summarized in Table 2 below. The blue roof is considerably hotter than a Gal-
valume roof. But this is a consistent measurement, and I attribute that to the color absorbing more heat. You also see that the heat is unable to transfer to the rafter, probably because of the standing seam roof and the improved insulation technique. This study was done in an unconditioned space, and the temperature of the building interior is still noticeably cooler than the outside environment. As an interesting aside, this new building
adjoined an existing building with the same fea- tures, except a “bag and sag” insulation system was used. This space was conditioned with air conditioners running full blast. We took the same temperature measurements with the results also shown in Table 2. It is apparent that the mechanical systems are
cooling the structural members, but they are still warmer than the liner system.
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